Automatic doors in buildings, and electric sliding doors for vehicles such as wagons and vans have an electromotive opening/closing device that slides a door by a motor or the like. Such electromotive opening/closing devices may pinch an object between a door frame and a door during door closing operation. Accordingly, electromotive opening/closing devices have been proposed that have control means detecting the pinching of an object and stopping a door closing operation, or changing the door closing operation to the door opening operation.
Japanese Patent No. 3300660 and Japanese Patent No. 3415014 disclose an automatic opening/closing device that provides pinching detecting function such as a door panel of an electric sliding door for vehicle has pinched an object between the moving door panel and a door frame. The automatic opening/closing device disclosed in each of these prior arts has a pressure sensitive sensor for detecting a pressing force caused by pinching of an object. This pressure sensitive sensor is formed by providing a cross hole inside an outer skin portion of a long tubes formed of an elastic material. The cross hole is gradually displaced around the center of the tube in the longitudinal direction of the tube and a plurality of conductive wires serving as electrodes are placed so as to be spaced from one another through the cross hole in the vicinity of the center of the cross hole, in a spiral manner along the cross hole.
Once the pressure sensitive sensor has been subjected to a pressing force by pinching of an object, the outer skin portion is elastically deformed, and thereby the cross hole inside the outer skin portion is squashed. Any or all of the plurality of conductive wires along the cross hole contact one another and make electric continuity therebetween. Thus, the pressure sensitive sensor detects pinching of the object by detecting this electric continuity.
However, when attempting to arrange this pressure sensitive sensor along a door frame, and the flexibility in the installation may be restricted because of pressure applied bending or setting undergoing the sensor itself.
A possible countermeasure against this problem would be to use a pressure sensitive sensor using a piezoelectric element, instead of a pressure sensitive sensor with the above-described conductive wires as a detection principle. The piezoelectric element detects an external force that is being applied, not based on the physical contact between conductive wires inside the sensor as in the pressure sensitive sensor in the above-described prior arts, but based on electric polarization generated in response to an external force (stress). Therefore, the pressure sensitive sensor using a piezoelectric element allows installation to a variety of places irrespective of installing methods.
Also, this type of pressure sensitive sensor starts to generate a voltage from a time when an external force is weak, such as an initial stage of a pinching process, an early detection is feasible. Japanese Patent Application Publication No. 2003-106048 discloses a technique by which this type of pressure sensitive sensor is applied to a hatchback door of an automobile.
In general, the piezoelectric element varies detection voltage significantly in response to variations of ambient temperature. Therefore, when it is used for pinching detection for an automatic door of a building, the detection voltage would vary due to variations of air temperature in seasons, variations of air temperature in a day, variations of ambient temperature due to solar irradiation, and so on. In particular, when attempting to apply the pressure sensitive sensor using this piezoelectric element for pinching detection of a sliding door for vehicle, the variation range of ambient temperature of working condition becomes more significant. Thus, the variations in detection voltage due to ambient temperature causes a serious problem.